The fuel cell is a power generation device that derives electric energy by electrochemically oxidizing fuel such as hydrogen and methanol, and in recent years, it has attracted attention as a clean energy source. Among others, the polymer electrolyte fuel cell, which normally works at a low operating temperature around 100° C. and has a high energy density, is expected to serve in a wide variety of fields as relatively small distributed power generation facilities and power generation equipment for movable bodies such as automobiles and ships. Furthermore, it has attracted attention as power source for small movable devices and portable appliances, and in particular it is expected to replace secondary batteries such as nickel hydrogen batteries and lithium ion batteries incorporated in portable telephones and personal computers.
A fuel cell commonly includes cells as units, each composed mainly of a membrane electrode assembly (hereinafter occasionally abbreviated as MEA) sandwiched between separators. A MEA consists mainly of electrodes, i.e., an anode and a cathode where the power generating reaction takes place, and a polymer electrolyte membrane that works to conduct protons between the anode and the cathode. A polymer electrolyte membrane is formed primarily of a polymer electrolyte material. Polymer electrolyte materials have been used also as, for example, binders for electrode catalyst layers. Polymer electrolyte membranes are required primarily to have high proton conductivity, and in particular, they must have high proton conductivity even under high temperature, low humidify conditions. Furthermore, polymer electrolyte membranes are required to be low in permeability to fuels so as to function as a barrier to prevent direct reaction between fuels and oxygen. Other required characteristics include chemical stability for resisting an oxidizing atmosphere during fuel cell operation, as well as mechanical strength and physical durability for resisting thin film formation and repeated swelling-drying cycles.
Conventionally, Nafion (registered trademark) (manufactured by DuPont), which is a perfluorosulfonic acid based polymer, has been used as material for polymer electrolyte membrane. Being manufactured through a multi-stage synthesis process, Nafion (registered trademark) is very high in price and it also has the problem of large fuel crossover (fuel permeability). It has been also pointed out that the product has other problems such as a decrease in film's mechanical strength and physical durability caused by swelling-drying cycles, inability to work at high temperatures due to low softening point, necessity of disposal treatments after use, and difficulty in recycling of materials.
Under such circumstances, active studies have been carried out in recent years to develop hydrocarbon based electrolyte membranes as polymer electrolyte materials that are so low in price and good in film characteristics as to replace Nafion (registered trademark).
For instance, some studies have proposed the use of a block copolymer composed mainly of hydrophobic segments virtually free of sulfonic acid groups and hydrophilic segments containing sulfonic acid group in which the hydrophobic segments include polyethersulfone (PES) or polyether ketone while the hydrophilic segments include sulfonated polyethersulfone or sulfonated polyether ketone (patent documents 1 and 2).
Patent document 3 describes an attempt of block copolymerization incorporating a small amount of 4,4′-dihydroxy benzophenone which contains two sulfonic acid groups. Non-patent document 1 describes the use of a block copolymer composed mainly of polyethersulfone (PES) as hydrophobic segment and sulfonated polyethersulfone in which phenyl groups contain a sulfonic acid group as hydrophilic segment.